Thermal exchange of the fuel elements in nuclear reactor



J- LE FOLL April 20, 1965 THERMAL EXCHANGE OF THE FUEL ELEMENTS IN NUCLEAR REACTOR Filed Aug. 1. 1960 United States Patent 3,179,570 THERMAL EXCHANGE OF THE FUEL ELEMENTS IN NUCLEAR REACTOR Jean Le Foll, Sevres, France, assignor to Commissariat a lEnergie Atomique, Paris, France Filed Aug. 1, 1966, Ser. No. 46,491 Claims priority, application France, Aug. 13, 1%9, 802,766; Apr. 6, 1960, 823,516 3 Claims. (Cl. 176-59) The present invention relates to an apparatus for inducing transverse currents of cooling fluid along the fuel elements of atomic reactors.

Improvement of thermal exchange between the body (whether or not encased) of the fuel elements or cartridges for nuclear reactors and the fluid which surrounds it is generally obtained by means of fins extending from the body. The fluid, most usually a compressed gas, circulates inside a channel containing the fuel element and the general direction of fluid flow is longitudinal with respect to the element.

In this way, the fins increase the thermal exchange surface. However, in order to be eflicient, they should be sufficiently short for the fall in temperature from the base to the tip of the fin to be small; on the other hand, local convection currents should be set up which improve the thermal exchange. In order to increase the exchange surface without lengthening the fins too much, it has been the practice to place the fins close to one another, but then a new difficulty appears.

In fact, by the use of very particular aerodynamic arrangements, high rates of flow only arise in the annular region situated between the tips of the fins and the channel. In contrast, although the elementary channels delimited by the fins are open to the aforesaid annular region, the rates there remain moderate and the local coefficients of exchange remain very limited immediately away from the tips of the fins.

To avoid this disadvantage, which becomes particularly serious in the case of very close-packed longitudinal fins, a device referred to by its inventor as a polyzonal cartridge has been used which allows a considerable increase of the exchange surface to be obtained. The fins are helicoidal and are cut by longitudinal partitions or splitters; the fall of pressure of the principal fluid flow in the main channel between the ends of the elementary channels defined by the fins is then small compared with the dynamic pressure of this same flow; the movement of the fluid between the fins is essentially an ascending rotary movement; furthermore, exchanges constantly take place between the fluid circulating in the elementary channels located between the fins and the fluid circulating in the main channel, which is obtained by means of the splitters which set up a considerable flow in the subsidiary channels and cause the fluid circulating in these subsidiary channels to be renewed periodically.

In contrast to what takes place in the case of a casing provided with longitudinal fins or helicoidal fins without splitters, the thermal etficiency of polyzonal casings increases according to the fineness and closeness of packing of the vanes and the only limit appears to be of a technological nature.

Circumferential fins have been used which present their faces perpendicularly to the flow of the fluid in the annular part. The arrangement usually adopted is that the fins are separated by a distance substantially equal to their height.

It is hence seen that a powerful dynamic and turbulent flow is set up between these fins, considerably increasing not only the heat exchange, but also the losses of the charge. The improvement obtained by means of this device is limited by the fact that the ting cannot be placed too closely together without causing the disappearance of the phenomenon described above which favours thermal exchange. Indeed, if the fins are placed transversely very closely together, sufliciently rapid local currents cannot be induced between the fins, that is to say, transversely of the body of the fuel element; eddy currents are only found at the tips of the fins and there is furthermore the disadvantage of an increased loss of charge along the channel.

The present invention consists essentially in providing transverse fins on the body of a fuel element, the thickness of the fins and the interval between them being made as small as the present technique of manufacture allows, the height of the fins being about 3-10 times the separation between them, and also in producing, along the circular transverse channels defined by the fins, a pressure field having one or more maxima, the field being obtained from the dynamic pressure corresponding to the longitudinal movement of the principal fluid current, so that the cooling fluid is constrained to travel transversely and tangentially along the circular transverse channels.

This fiuid under pressure may be obatined by the following means.

On the exterior contour formed by the assembly of transverse fins, wing members are provided having the form of flat arrowheads constituting triangular blades attached to a backbone fixed longitudinally to the casing of the fuel element following a generatrix; the plane of fixing of the blades forming an angle with the plane tangential to the casing along the generatrix, the sine of which is approximately the ratio of the thickness of the passages between the transverse fins to the sum of the thickness of these passages and the thickness of the fins.

This structure superimposed on the transverse fins allow a difference in propulsive pressure to be set up between the inlets and outlets of the elementary transverse channels defined by these fins.

Referring to FIGURES 1 to 4 of the accompanying drawings, an embodiment of devices for the production of transverse currents of fluid along the fuel elements of nuclear reactors, according to the invention, is described below, by Way of example.

The arrangement to be described in connection with this embodiment is to be considered as forming part of the invention, it being understood that all equivalent arrangements may be utilised without exceeding the scope thereof.

FIGURE 1 is a partial axial view of a casing of a fuel element comprising triangular wing members attached to a backbone fixed longitudinally to the casing, the plane of the section passing through a midrib;

FIGURE 2 is a side elevation of FIGURE 1;

FIGURE 3 is an end view of the casing of FIGURE 1;

FIGURE 4 is a free perspective representation of a broken piece of this same casing.

In these figures, only those elements necessary for the understanding of the invention have been depicted; corresponding elements in different figures carry the same references.

In the modification shown in FIGURES l to 4, defiective elements have a plane triangular or delta form and are designated by the reference 27. A backbone 28 which is long compared with its breadth, is secured to the transverse cooling fins 2 of the casing 1 of the rod 1a. The backbone 28 is fixed to the casing along a generatrix of the right cylinder enclosing the fins themselves. According to FIG. 3 and the embodiment chosen, the number of such backbones is equal to four. They are arranged symmetrically on the circumference of the casing and from them are derived in turn the planes of symmetry for the delta vanes 27.

The vanes 27 are then situated on both sides of each of the backbones 28. The inclination of the plane of these vanes 27 with respect to the plane tangential to the cylinder along the generatrix connected with a backbone 28 is determined so that these planes make an angle between them the since of which is preferably approximately the same or slightly greater than the ratio of the width of the spaces between the transverse fins 2 to the sum of the thickness of these spaces and the thickness of the fins. As an example of this embodiment, the thickness of the spaces being equal to that of the fins, the angle at which the vanes 27 are mounted is about 30.

These vanes and the backbone which supports them have in practice a purely aerodynamic role and only participate in heat exchanges with a very low coefiicient. Consequently, this assembly can be constructed in a material different from that of the casing, so far as the choice of such a material is advantageous as regards absorption of neutrons or techniques of construction. For example, the inductive assembly (delta vanes and backbone) can be made from graphite, which has the advantage of acting as a moderator and absorbing few neutrons. In this case and according to FIGURE 1, the backbones 28 can be prolonged as far as the channel of the pile and then are supported by lugs 29 on the Walls 10 of the channel and thus ensure the centering of the fuel element cartridge in the reactor channel.

Another modification according to the invention consists in cutting down the triangular vanes on a part of their length so as to avoid marginal turbulence of the fluid on these vanes and to suppress useless frothing of liquid under the sides of the triangular vanes.

The principal flow of cooling fluid is shown by the arrow 30 (FIGS. 1 and 4). The arrow 31 represents the path of a portion of this same fluid subjected to deflection by the vanes 27; it is seen that this path is transverse as far as the point 32 and then passes down the space between the fins 2.

I claim:

1. In a nuclear reactor fuel channel, the combination of a nuclear reactor fuel element and a device for removing heat from the nuclear fuel element having a sheath provided with transverse cooling fins by means of a cooling fluid so as to make the fluid circulate transversely throughout the unitary circular channels formed by the fins, a plurality of spaced deflecting elements uniformly distributed over the fins for deflecting the fluid into the circular channels, each of said elements being triangular with one side tangent to the edges of the fins and lying in a single plane oblique to the longitudinal axis of the fuel element and forming arrow-shaped flat vanes, said triangular elements down toward the fins, and longitudinal backbones parallel to the longitudinal axis of the fuel element secured to and in contact with the outer edges of the fins supporting said deflecting elements, the longitudinal spacing of said deflecting elements being such that a cross section transverse to the longitudinal axis of the fuel elements includes two of said deflecting elements supported by the same backbone.

2. Device as described in claim 1 in which the angle between a plane tangent to the fins along one of said backbones and the plane of said vanes has its sine approaching the ratio of the distance between adjacent transverse fins and the sum of said distance and the thickness of a fin.

3. Device as described in claim 1, said backbones including lugs extending adjacent to the inner wall of the channel containing the fuel element and bearing on the wall for centering the fuel element.

References Cited by the Examiner UNITED STATES PATENTS OTHER REFERENCES Printed application No. 1,066,291, Germany.

October 1959,

45 CARL D. QUARFORTH, Primary Examiner.

OSCAR R. VERTIZ, ROGER L. CAMPBELL,

Examiners.

UNITE-D STATES-PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,179,570 April 20, 1965- Jean Le Foll It is hereby certified that error appears in-the above numbered bat- I ent reqliring correction and that the said Letters Patent should read as correctedbelow.

Column 3, line 6, for "since" read sine Signed and sealed this 17th day of August 1965 (sum Attest:

EDWARD J. BRENNER Commissioner of Patents' ERNEST w. SWIDER -All.csting Officer UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,179,570 I April 20, 1965 Jean Le F011 It is hereby certified that error appears in the above numbered batj ent reqliring correction and that the said Letters Patent should read as corrected below Column 3 line 6 for "since" read sine Signed and sealed this 17th 'dayof August 1965 SEA L) Attest:

EDWARD J. BRENNER Commissioner of Patents ERNEST w. SWIDER Al testing Officer 

1. IN A NUCLEAR REACTOR FUEL CHANNEL, THE COMBINATION OF A NUCLEAR REACTOR FUEL ELEMENT AND A DEVICE FOR REMOVING HEAT FROM THE NUCLEAR FUEL ELEMENT HAVING A SHEATH PROVIDED WITH TRANSVERSE COOLING FINS BY MEANS OF A COOLING FLUID SO AS TO MAKE THE FLUID CIRCULATE TRANSVERSELY THROUGHOUT THE UNITARY CIRCULAR CHANNELS FORMED BY THE FINS, A PLURALTIY OF SPACED DEFLECTING ELEMENTS UNIFORMLY DISTRIBUTED OVER THE FINS FOR DEFLECTING THE FLUID INTO THE CIRCULAR CHANNELS, EACH OF SAID ELEMENTS BEING TRIANGULAR WITH ONE SIDE TANGENT TO THE EDGES OF THE FINS AND LYING IN A SINGLE PLANE OBLIQUE TO THE LONGTUDINAL AXIS OF THE FUEL ELEMENT AND FORMING ARROW-SHAPED FLAT VANES, SAID TRIANGULAR ELEMENTS DOWN TOWARD THE FINS, LONGITUDINAL BACKBONES PARALLEL TO THE LONGITUDINAL AXIS 